This invention relates to a wide band chrominance demodulator for a color TV receiver etc., particularly to a wide band chrominance demodulator with color noise reduction function enabling to reduce color noise at weak field strength.
Generally, in a composite color TV signal of NTSC system or the like, a carrier chrominance signal and a luminance signal are multiplexed by a frequency interleaving system, thereby the compatibility of color and monochrome TV receivers is established and a most effective data transmission within a restricted frequency range already assigned is ensured. For the multiplex transmission with frequency interleaving, the chrominance subcarrier frequency for transmitting color difference signals is selected to be an odd number multiple of half of the horizontal scanning frequency, namely the chrominance subcarrier frequency is 3.579545 MHz (hereinafter referred to as 3.58 MHz). Two carrier chrominance signals are respectively obtained from the chrominance subcarrier and two color difference signals by the manner that the chrominance subcarrier is subjected to a suppressed carrier amplitude modulation with the two color difference signals. These carrier chrominance signals are added to the luminance signal to generate a frequency-interleaved signal, and such frequency-interleaved signal is transmitted.
Human visual characteristics depend on kind of color. A human color vision is sensitive to orange-cyanic color and is relatively insensitive to green-magenta color. According to this, the color difference signal of orange-cyanic color is transferred as an I signal having a frequency range of 0-1.5 MHz (wide band), whereas the color difference signal of green-magenta color is transferred as a Q signal having a frequency range of 0-0.5 MHz (narrow band). These I and Q signals are used for suppressed-carrier-amplitude-modulating the chrominance subcarrier.
Generally, in an analog transmission system a S/N ratio over 3 MHz is of quite low grade. Since the frequency of chrominance subcarrier is high, i.e. 3.58 MHz, color noise, which is caused mainly by white noise at weak field strength, remarkably increases. Particularly, in a wide band chrominance demodulator of IQ demodulation system, since the S/N ratio of high frequency components of I signal (wide band) is liable to degrade, color noise, e.g. color snow, is considerably in the forefront. Such noise degradation will be explained referring to FIG. 1 as follows.
In FIG. 1 a comb filter 1 includes a delay circuit for one horizontal scanning period (a 1H delay circuit) 2, an adder 3 and a subtractor 4. The 1H delay circuit 2 may be a glass delay line (ultrasonic delay line) or a CCD delay circuit. A composite color video signal is applied via a signal supply terminal 5 to an input terminal of filter 1. The composite color video signal is inputted to respective circuit elements 2, 3 and 4. An output signal of delay circuit 2 is applied to adder 3 and subtractor 4. Adder 3 outputs a luminance signal component, and subtractor 4 outputs a chrominance signal component. The output signal of adder 3 is supplied to a video amplifier 7 through a low pass filter (LPF) 6 having a cutoff frequency of 3.58 MHz. Video amplifier 7 provides a luminance signal Y.
The output signal of subtractor 4 is inputted to a band pass filter (BPF) 9. BPF 9 permits to pass therethrough frequency components of 3.58+0.5/-1.5 MHz. A filtered output signal of BPF 9 or a carrier chrominance signal is inputted to an I axis demodulator 10 and a Q axis demodulator 11. An I signal obtained from I demodulator 10 is applied to a color matrix circuit 8 via a LPF 12 having 1.5 MHz cutoff and a delay line 13. A Q signal obtained from Q demodulator 11 is applied via a LPF 14 having 0.5 MHz cutoff to matrix circuit 8.
The wide band chrominance demodulator shown in FIG. 1 will operate as follows.
A composite color video signal occupies a frequency range of DC-4.2 MHz. Within this range, a RF carrier is carrier-suppressed-modulated by Y signal, and a chrominance subcarrier whose frequency is deviated by 3.58 MHz from the RF carrier frequency is also carrier-suppressed-modulated by two color difference signals, i.e. I and Q signals. The modulated Y signal and chrominance subcarrier are frequency-interleaved and then transmitted, as already mentioned. A Y signal component of the composite color video signal is obtained from the output terminal of adder 3. Useless frequency components being attendant on the Y signal are filtered out through LPF 6. Video amplifier 7 amplifies the filtered Y signal to provide the luminance signal Y.
A chrominance signal component of the composite color video signal is obtained from the output terminal of subtractor 4. Useless frequency components of the chrominance signal are filtered out by BPF 9 having a pass-band of about 2.1-4.1 MHz. BPF 9 provides I and Q demodulators 10 and 11 with the filtered carrier chrominance signal. I demodulator 10 demodulates the I signal component of inputted chrominance signal to provide a demodulated I signal. The demodulated I signal is applied to matrix circuit 8 via LPF 12 having 1.5 MHz cutoff and delay line 13 for compensating the group delay time discrepancy of I and Q signals. Q demodulator 11 demodulates the Q signal component of inputted chrominance signal to provide a demodulated Q signal. The demodulated Q signal is applied to LPF 14 having 0.5 MHz cutoff. LPF 14 provides matrix circuit 8 with Q signal being free from useless frequency components. Matrix circuit 8 composes color difference signals (R-Y), (G-Y) and (B-Y) from inputted I and Q signals.
In the chrominance demodulator as shown in FIG. 1, since a transmission frequency range of I signal covers the maximum frequency of transmitted I signal, i.e. 1.5 MHz, the color resolution will be best. This is satisfactory when a strong field strength is available. However, when the field strength is weak, color noise caused mainly by white noise of higher frequency (.gtorsim.0.5 MHz) increases to a high degree. Although the color noise may be reduced by restricting the frequency range of chrominance signal (e.g. 0.5 MHz or less), such frequency restriction causes the TV set to provide "chrominance smear" and thus to degrade the picture quality.